Wrapping a ListStore with an Rc no longer (?) works anymore

[cmdcolin]

I was trying to update a sample app that worked with 0.5.x using a ListStore in a Rc. Example code similar to the column_view_datagrid example

setup

    let store = gio::ListStore::new(BoxedAnyObject::static_type());

    (0..10000).for_each(|i| {
        store.append(&BoxedAnyObject::new(Row {
            col1: format!("col1 {i}"),
            col2: format!("col2 {i}"),
        }))
    });

fails with compile error

    let store_rc = Rc::new(store);
    let sel = gtk::SingleSelection::new(Some(&store_rc)); // fails with compile error
    let columnview = gtk::ColumnView::new(Some(&sel))

works

    let sel = gtk::SingleSelection::new(Some(&store)); // works
    let columnview = gtk::ColumnView::new(Some(&sel))

the compile error is

error[E0277]: the trait bound `Rc<gtk4::gio::ListStore>: IsA<ListModel>` is not satisfied
  --> examples/column_view_datagrid/main.rs:44:41
   |
44 |     let sel = gtk::SingleSelection::new(Some(&store_rc));
   |               ------------------------- ^^^^^^^^^^^^^^^ the trait `IsA<ListModel>` is not implemented for `Rc<gtk4::gio::ListStore>`
   |               |
   |               required by a bound introduced by this call
   |
   = help: the following other types implement trait `IsA<T>`:
             <ATContext as IsA<ATContext>>
             <ATContext as IsA<gtk4::glib::Object>>
             <AboutDialog as IsA<AboutDialog>>
             <AboutDialog as IsA<Accessible>>
             <AboutDialog as IsA<Buildable>>
             <AboutDialog as IsA<ConstraintTarget>>
             <AboutDialog as IsA<Native>>
             <AboutDialog as IsA<Root>>
           and 1763 others
note: required by a bound in `SingleSelection::new`
  --> /home/cdiesh/src/gtk4-rs/gtk4/src/auto/single_selection.rs:29:36
   |
29 |     pub fn new(model: Option<&impl IsA<gio::ListModel>>) -> SingleSelection {
   |                                    ^^^^^^^^^^^^^^^^^^^ required by this bound in `SingleSelection::new`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `gtk4-rs-examples` due to previous error

this used to work for me but after i tried updating to 0.6.0 started giving the above error.

here is an example PR for attempting the 0.6.0 upgrade that producing this error

https://github.com/cmdcolin/fml9000/pull/17/files

trying rustc 1.69.0-nightly (2773383a3 2023-02-10) but also was affecting rustc 1.68.0-nightly (77429957a 2023-01-01)

[sdroege]

Why do you want to wrap it in an Rc? The ListStore already behaves like an Rc: clone() will simply increase its reference count.

For your code, it will probably work if you do store_rc.as_ref() but having two layers of reference counting is not a good idea.

[cmdcolin]

thanks i think i fixed it without using an Rc now. that Rc was probably unneeded, i'm quite new to rust and still have a pretty weak grasp on the memory management and sometimes wrap things in Rc just to avoid some of the compiler complaints

I managed to fix the PR using just clone and I think it is working again

here is my updated code if there is interest, quite naive approaches all around in the code probably

cmdcolin/fml9000#17

[sdroege]

Seems fine to me. You might be able to get rid of some of the clone()s depending on the surrounding code but that doesn't really matter much for reference counting.

Knowing whether a type in Rust does deep copies or refcounting or something else is indeed not obvious.